Hi @mcoy,
With the laptop on, try shining a flashlight at an angle close to the screen and check if you can see any images at all. They will be ''very faint'' if there so trying this in a darkened room may help to see them.
''If you can see images'' then there a backlight problem with the display. Since you have tried different presumably known working display panels and display cables then as you say there's a problem on the motherboard.
What is the motherboard's model number as printed on the motherboard, maybe GA0nnnnnn? The schematics will be needed to be found to find out how the power is delivered to the panel through the cable. Some motherboards have a backlight fuse whereas others supply directly from a power mosfet.
Nowadays what is applicable to both though is that there's always backlight power supplied to the screen but it is turned on and off in the screen by a signal sent on the backlight enable lead from the motherboard to the panel.
''If you can't see any images'' and if you have a display device that has a USB Type-C connector, providing DisplayPort output then connect an external monitor to either of the laptop's USB-C ports and check if there's a display showing on it.